How Magnets Work

Many of today's electronic devices require magnets to function. This reliance on magnets is relatively recentprimarily because most modern devices require magnets that are stronger than the ones found in nature. Lodestonea form of magnetiteis the strongest naturally occurring magnet. It can attract small objectslike paper clips and staples.

By the 12th centurypeople had discovered that they could use lodestone to magnetize pieces of ironcreating a compass. Repeatedly rubbing lodestone along an iron needle in one direction magnetized the needle. It would then align itself in a north-south direction when suspended. Eventuallyscientist William Gilbert explained that this north-south alignment of magnetized needles was due to Earth behaving like an enormous magnet with north and south poles.

A compass needle isn't nearly as strong as many of the permanent magnets used today. But the physical process that magnetizes compass needles and chunks of neodymium alloy is essentially the same. It relies on microscopic regions known as magnetic domainswhich are part of the physical structure of ferromagnetic materialslike ironcobalt and nickel. Each domain is essentially a tinyself-contained magnet with a north and south pole. In an unmagnetized ferromagnetic materialeach domain's north pole points in a random direction. Magnetic domains that are oriented in opposite directions cancel one another outso the material does not produce a net magnetic field.

In magnetson the other handmost or all the magnetic domains point in the same direction. Rather than canceling one another outthe microscopic magnetic fields combine to create one large magnetic field. The more domains point in the same directionthe stronger the overall field. Each domain's magnetic field extends from its north pole into the south pole of the domain ahead of it.

This explains why breaking a magnet in half creates two smaller magnets with north and south poles. It also explains why opposite poles attract — the field lines leave the north pole of one magnet and naturally enter the south pole of anotheressentially creating one larger magnet. Like poles repel each other because their lines of force are traveling in opposite directionsclashing with each other rather than moving together.

To make a permanent magnetall you have to do is encourage the magnetic domains in a piece of metal to point in the same direction. That's what happens when you rub a needle with a magnet — the exposure to the magnetic field encourages the domains to align. Other ways to align magnetic domains in a piece of metal include:

Two of these methods are among scientific theories about how lodestone forms in nature. Some scientists speculate magnetite becomes magnetic when struck by lightning. Others theorize that pieces of magnetite became magnets when Earth was first formed. The domains aligned with Earth's magnetic field while iron oxide was molten and flexible.

The most common method of making magnets today involves placing metal in a magnetic field. The field exerts torque on the materialencouraging the domains to align. There's a slight delayknown as hysteresisbetween the application of the field and the change in domains; it takes a few moments for the domains to start to move. Here's what happens:

The resulting magnet's strength depends on the amount of force used to move the domains. Its permanenceor retentivitydepends on how difficult it was to encourage the domains to align. Materials that are hard to magnetize generally retain their magnetism for longer periodswhile materials that are easy to magnetize often revert to their original nonmagnetic state.

You can reduce a magnet's strength or demagnetize it entirely by exposing it to a magnetic field that is aligned in the opposite direction. You can also demagnetize a material by heating it above its Curie pointor the temperature at which an object's magnetic properties change. The heat distorts the material and excites the magnetic particlescausing the domains to fall out of alignment.

If you've read How Electromagnets Workyou know that an electrical current moving through a wire creates a magnetic field. Moving electrical charges are responsible for the magnetic field in permanent magnets as well. But a magnet's field doesn't come from a large current traveling through a wire — it comes from the movement of electrons.

Many people imagine electrons as tiny particles that orbit an atom's nucleus the way planets orbit a sun. As quantum physicists currently explain itthe movement of electrons is a little more complicated than that. Essentiallyelectrons fill an atom's shell-like orbitalswhere they behave as both particles and waves. The electrons have a charge and a massas well as a movement that physicists describe as spin in an upward or downward direction.

Generallyelectrons fill the atom's orbitals in pairs. If one of the electrons in a pair spins upwardthe other spins downward. It's impossible for both of the electrons in a pair to spin in the same direction. This is part of a quantum-mechanical principle known as the Pauli Exclusion Principle.

Even though an atom's electrons don't move very fartheir movement is enough to create a tiny magnetic field. Since paired electrons spin in opposite directionstheir magnetic fields cancel one another out. Atoms of ferromagnetic elementson the other handhave several unpaired electrons that have the same spin. Ironfor examplehas four unpaired electrons with the same spin. Because they have no opposing fields to cancel their effectsthese electrons have an orbital magnetic moment. The magnetic moment is a vector — it has a magnitude and a direction. It's related to both the magnetic field strength and the torque that the field exerts. A whole magnet's magnetic moments come from the moments of all of its atoms.

In metals like ironthe orbital magnetic moment encourages nearby atoms to align along the same north-south field lines. Iron and other ferromagnetic materials are crystalline. As they cool from a molten stategroups of atoms with parallel orbital spin line up within the crystal structure. This forms the magnetic domains discussed in the previous section.

You may have noticed that the materials that make good magnets are the same as the materials magnets attract. This is because magnets attract materials that have unpaired electrons that spin in the same direction. In other wordsthe quality that turns a metal into a magnet also attracts the metal to magnets. Many other elements are diamagnetic — their unpaired atoms create a field that weakly repels a magnet. A few materials don't react with magnets at all.

This explanation and its underlying quantum physics are fairly complicatedand without them the idea of magnetic attraction can be mystifying. So it's not surprising that people have viewed magnetic materials with suspicion for much of history.

Every time you use a computeryou're using magnets. If your home has a doorbellit probably uses an electromagnet to drive a noisemaker. Magnets are also vital components in CRT televisionsspeakersmicrophonesgeneratorstransformerselectric motorsburglar alarmscassette tapescompasses and car speedometers.

In addition to their practical usesmagnets have numerous amazing properties. They can induce current in wire and supply torque for electric motors. Maglev trains use magnetic propulsion to travel at high speedsand magnetic fluids help fill rocket engines with fuel.

Earth's magnetic fieldknown as the magnetosphereprotects it from the solar wind. According to Wired magazinesome people even implant tiny neodymium magnets in their fingersallowing them to detect electromagnetic fields.

Magnetic resonance imaging (MRI) machines use magnetic fields to allow doctors to examine patients' internal organs. Doctors also use pulsed electromagnetic fields to treat broken bones that have not healed correctly. This methodapproved by the United States Food and Drug Administration in the 1980scan mend bones that have not responded to other treatment. Similar pulses of electromagnetic energy may help prevent bone and muscle loss in astronauts who are in microgravity environments for extended periods.

Magnets can also protect the health of animals. Cows are susceptible to a condition called traumatic reticulopericarditisor hardware diseasewhich comes from swallowing metal objects. Swallowed objects can puncture a cow's stomach and damage its diaphragm or heart. Magnets are instrumental to preventing this condition.

One practice involves passing a magnet over the cows' food to remove metal objects. Another is to feed magnets to the cows. Longnarrow alnico magnetsknown as cow magnetscan attract pieces of metal and help prevent them from injuring the cow's stomach.

Peopleon the other handshould never eat magnetssince they can stick together through a person's intestinal wallsblocking blood flow and killing tissue. In humansswallowed magnets often require surgery to remove.

Some people advocate the use of magnet therapy to treat a wide variety of diseases and conditions. According to practitionersmagnetic insolesbraceletsnecklacesmattress pads and pillows can cure or alleviate everything from arthritis to cancer. Some advocates also suggest that consuming magnetized drinking water can treat or prevent various ailments.

Proponents offer several explanations for how this works. One is that the magnet attracts the iron found in hemoglobin in the bloodimproving circulation to a specific area. Another is that the magnetic field somehow changes the structure of nearby cells.

Howeverscientific studies have not confirmed that the use of static magnets has any effect on pain or illness. Clinical trials suggest that the positive benefits attributed to magnets may actually come from the passage of timeadditional cushioning in magnetic insoles or the placebo effect. In additiondrinking water does not typically contain elements that can be magnetizedmaking the idea of magnetic drinking water questionable.